Seems the more I read the more confused I get, I understood that the 4 inch and biger exhaust are the way to go, and you wanted to keep the EGT's cool and then I read this on the flomaster sight. I am really confused what to do? Here is the post that I read....

"A maximum of 1200°F at the exhaust manifold before the turbo. A maximum of 950°F after the turbo. There is a difference of 250°F from before the turbo to after the turbo. It is very important where the temperature is measured. A Pyrometer is a temperature gauge for reading exhaust gas temperature (EGT.)"

Usmc 1775
2001 psd 4x4 cc wanting to up grade only if I would quit reading things

 

It looks like what they're trying to say is that you don't need a bigger diamter pipe because the exhaust temp cools down and becomes less dense, therefor decreasing backpressure. This is true, on a non-turbo vehicle. But luckily you've got a turbo on yours The turbo gives you all the backpressure you need, so you can run as big a pipe as your heart desires.